VoIP (Voice over Internet Protocol) is one of the popular communication technology. In VoIP, SIP (Session Initiation Protocol) defined by IETF is the most widely used protocol because of its simple structure, expandability and easy operation.
In a conventional telephone system, the number of extensions is limited by the capacity of the Private Branch Exchange. A Private Branch Exchange with more capacity will have a higher price. Therefore it is often to combine an Internet telephone system into a conventional telephone system for increasing the extensions and for upgrading into an IP telephone system so as to provide a more flexible expandability for extensions.
FIG. 1 shows a conventional IP telephone system, which is designed in an elementary school. The Internet telephone system comprises an SIP proxy server 1 and Internet extensions 10. The conventional telephone system comprises a gateway 2 of conventional telephone system, a Private Branch Exchange (PBX) 3, an auto attendants 4 and conventional extensions 30. The SIP proxy server 1 of the Internet telephone system connects with an upper SIP proxy server 6 of Taipei Education Bureau. The gateway 2 of conventional telephone system connects with Chung-Hwa Telecommunication Co. 5. SIP proxy server 1 connects with Internet extensions 10, auto attendants 4, gateway 2 and SIP proxy server 6 by Internet indicated by dotted lines. Chung-Hwa Telecommunication Co. 5, gateway 2, Private Branch Exchange 3, auto attendants 4 and conventional extensions 30 are connected by conventional telephone lines indicated by solid lines, as shown in the figure.
A dialing from an Internet extension 10 to a conventional extension 30 has to dial the telephone number of the Private Branch Exchange 3 first so as to pass through SIP proxy server 1, gateway 2, Private Branch Exchange 3, and then answered by the auto attendants 4 for providing a voice guidance, thereafter dial the number 30 of the conventional extension 30 so that a connection between the Private Branch Exchange 3 and the conventional extension 30 is established, as shown by the bold solid arrow lines in FIG. 1
The dialing described above has to dial the telephone number of the Private Branch Exchange 3 first and then directed by a voice guidance, it is not convenient.